Thortveitite-type Tm₂Si₂O₇

Abstract

Single crystals of dithulium disilicate, Tm₂Si₂O₇, were obtained in flux synthesis experiments in the system SiO₂–Tm₂O₃–LiF at ambient pressure. The compound belongs to the group of sorosilicates, i.e. it is based on [Si₂O₇]-units and crystallizes in the thortveitite (Sc₂Si₂O₇) structure type. The Tm³⁺ cation (site symmetry .2.) occupies a distorted octa­hedral site, with Tm—O bond lengths in the range 2.217 (4)–2.289 (4) Å. Each of the octa­hedra shares three of its edges with adjacent [TmO₆] groups, resulting in the formation of layers parallel to (001). The individual [SiO₄] tetra­hedra are more regular, i.e. the differences between the bond lengths between Si and the bridging and non-bridging O atoms are not very pronounced. The layers containing the octa­hedra and the sheets containing the [Si₂O₇] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.

Related literature  

For applications of oxosilicates containing trivalent rare earth elements (REE), see: Kitai (2008 ▶); Piccinelli et al. (2009 ▶); Qiao et al. (2014 ▶); Luo et al. (2012 ▶); Streit et al. (2013 ▶); Han et al. (2006 ▶); Sun et al. (2012 ▶). For structures isotypic with that of the title compound, see: Zachariasen (1930 ▶); Smolin et al. (1973 ▶); Christensen (1994 ▶); Redhammer & Roth (2003 ▶). For polymorphic forms of Tm₂Si₂O₇ and other structure types adopted by (REE)₂Si₂O₇ compounds, see: Bocquillon et al. (1977 ▶); Hartenbach et al. (2003 ▶); Felsche (1973 ▶); Fleet & Liu (2005 ▶); Shannon & Prewitt (1970 ▶). For discussions of the [Si₂O₇]-unit with a linear bridging angle, see: Baur (1980 ▶); Bianchi et al. (1988 ▶); Cruickshank et al. (1962 ▶); Kimata et al. (1998 ▶); Liebau (1961 ▶). For general aspects on the crystal chemistry of silicates, see: Liebau (1985 ▶). For definition of distortion parameters, see: Robinson et al. (1971 ▶). For bond-valence analysis, see: Brown (2002 ▶). For definition and calculation of similarity descriptors, see: Tasci et al. (2012 ▶); Bergerhoff et al. (1999 ▶). For ionic radii, see: Shannon (1976 ▶). For the Inorganic Crystal Structure Database, see: ICSD (2014 ▶).

Experimental  

Crystal data  

• Tm₂Si₂O₇

• M _r = 506.04

• Monoclinic,

• a = 6.8205 (14) Å

• b = 8.9062 (18) Å

• c = 4.6937 (11) Å

• β = 101.78 (2)°

• V = 279.11 (10) ų

• Z = 2

• Mo Kα radiation

• μ = 31.99 mm⁻¹

• T = 293 K

• 0.05 × 0.03 × 0.01 mm

Data collection  

• Agilent Xcalibur (Ruby, Gemini ultra) diffractometer

• Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2014 ▶) T _min = 0.231, T _max = 1

• 894 measured reflections

• 340 independent reflections

• 330 reflections with I > 2σ(I)

• R _int = 0.020

Refinement  

• R[F ² > 2σ(F ²)] = 0.018

• wR(F ²) = 0.045

• S = 1.15

• 340 reflections

• 32 parameters

• Δρ_max = 1.62 e Å⁻³

• Δρ_min = −1.42 e Å⁻³

Data collection: CrysAlis PRO (Agilent, 2014 ▶); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR2002 (Burla et al., 2003 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: ATOMS for Windows (Dowty, 2011 ▶); software used to prepare material for publication: publCIF (Westrip, 2010 ▶) and WinGX (Farrugia, 2012 ▶).

Supplementary Material

CCDC reference: 1006971

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

S1. Comment

Oxosilicates that contain trivalent rare earth elements have been studied frequently because of their potential usage in the field of luminescense including applications in devices and circuits for electronic, optoelectronic as well as communication industries (Kitai, 2008; Piccinelli et al., 2009; Qiao et al., 2014; Luo et al., 2012; Streit et al., 2013; Han et al., 2006; Sun et al., 2012).

In the course of an ongoing project on the synthesis of alkali-REE-silicates (REE is a rare earth element), single-crystals of Tm₂Si₂O₇ have been obtained and structurally characterized. Synthetic Tm₂Si₂O₇ is isotypic with thortveitite (Sc₂Si₂O₇), a rare scandium silicate mineral (Zachariasen, 1930; Smolin et al., 1973). The compound is a sorosilicate and contains isolated [Si₂O₇]-groups. The bridging oxygen atom of the dimer resides on a centre of symmetry, resulting in a linear Si—O—Si angle. The conformation of the group is staggered with a dihedral angle (or azimuth) of 60° (Fig. 1). In the past, the question whether or not Si—O—Si angles can exhibit a value of 180° has been discussed controversially and, actually, the thortveitite structure-type played an important role in this debate (Liebau, 1961; Cruickshank, et al., 1962). However, a critical analysis of published data performed by Baur (1980) revealed that linear Si—O—Si bridging angles do exist and cannot be attributed to incorrect space group assignments. To date, it is generally accepted that a description of the thortveitite structure-type in the centrosymmetric space group C2/m (implying a linear Si—O—Si angle) is correct (Bianchi et al., 1988; Kimata et al., 1998). The present structure determination of Tm₂Si₂O₇ also confirms this model. The spread in the Si—O and O—Si—O angles is not very pronounced and the values are in the expected limits for silicates (Liebau, 1985). Numerically, the degree of distortion can be expressed by the quadratic elongation QE and the angle variance AV (Robinson et al., 1971). The values of these distortion parameters for a single [SiO₄]-tetrahedron are very small: 1.001 (for QE) and 4.95 (for AV), respectively. The Tm³⁺ cations are octahedrally coordinated by O atoms (Fig. 2), with Tm—O bond lengths in the range 2.217 (4) – 2.289 (4) Å and an average of 2.247 Å. The mean value compares well with those observed for the thortveitite representatives of the directly neighbouring REE Yb (<Yb—O>=2.240 Å) and Er (<Er—O>=2.253 Å) (Christensen, 1994). The differences can be attributed to the increasing ionic radii of the trivalent cations in the series Yb³⁺ - Tm³⁺ - Er³⁺ (Shannon, 1976). The octahedra show a distortion with moderate QE-values (1.061) and very high values for the angle variance. The high AV value of 219.7 seems to be a characteristic feature of the thortveitite structure-type and has been also observed for other members of this family (Redhammer & Roth, 2003). Each of the octahedra shares three of its edges with adjacent [TmO₆]-groups resulting in the formation of layers parallel to (001). These pseudo-hexagonal sheets (Fig. 3) are similar to the layers in dioctahedral micas. The above-mentioned pronounced angular distortions can be rationalized by a combination of (i) a shortening of the common edges of adjacent octahedra (in order to reduce the repulsive interactions between adjoining Tm³⁺ cations) and (ii) a widening of the corresponding opposite unshared O—O edges. Successive layers containing octahedra are linked by the [Si₂O₇]-groups in such a way that each of both tetrahedra shares two of its corners with two different octahedra from the same and one corner with an octahedron from the other surrounding layer.

Bond valence sum calculations using the parameter sets for the Tm—O and Si—O bonds given by Brown (2002) resulted in the following values (in v.u.) for the cation-anion interactions up to 3.4 Å: Tm: 3.09, Si: 4.01, O1: 2.12, O2: 1.99 and O3: 1.99.

As mentioned above, the present structure is isotypic with that of thortveitite. For the calculation of several quantitative descriptors for the characterization of the degree of similarity between the crystal structures of Tm₂Si₂O₇ and Sc₂Si₂O₇, the program COMPSTRU (Tasci et al., 2012) was employed. For the given two structures, the degree of lattice distortion (S), i.e. the spontaneous strain obtained from the eigenvalues of the finite Lagrangian strain tensor calculated in a Cartesian reference system, has a value of (S) = 0.0222. After application of an origin shift of p = (0, 0, 1/2) the structure of Tm₂Si₂O₇ was transformed to the most similar configuration of Sc₂Si₂O_7. The calculations revealed the following atomic displacements (in Å) between the corresponding atoms in Sc₂Si₂O₇ (first entry) and Tm₂Si₂O₇ (second entry): Sc—Tm: 0.025; Si—Si: 0.043; O1—O2: 0.000; O2—O1: 0.083; O3—O3: 0.070 i.e. the maximum displacement is lower than 0.10 Å. The measure of similarity (Δ) as defined by Bergerhoff et al. (1999) has a value of 0.059.

Since the beginning of the 1970ies a large number of different structure types have been described for rare earth element silicates with composition (REE)₂Si₂O₇ (Felsche, 1973). To date, at least twelve different forms (A—I, K, L and X) have to be distinguished (Fleet & Liu, 2005). Tm₂Si₂O₇, for example, exhibits a high degree of polymorphism where five different modifications can be realised. The synthesis of polycrystalline Tm₂Si₂O₇ adopting the thortveitite- or C-type has been described by Bocquillon et al. (1977) in the temperature range between 1473 and 1673 K. However, the stability field of C-type Tm₂Si₂O₇ extends to higher pressures as well: synthesis runs performed at 65 kbar/1773 K (Shannon & Prewitt, 1970) as well as 10 kbar/973 K and 18 kbar/973 K (Bocquillon et al., 1977) also resulted in the formation of the C-phase. Other high-pressure modifications of Tm₂Si₂O₇ crystallize in the B–, D–, X– and L-types (Fleet & Liu, 2005; Shannon & Prewitt, 1970). The B-type, however, has been also prepared at ambient pressure and 1173 K (Hartenbach et al., 2003). In summary, one can say that more than forty years after the first systematic investigations to chart the p,T-behaviour of Tm₂Si₂O₇, there are still open questions. The new flux synthesis route using lithium fluoride as a mineralizer offers the possibility to grow large single-crystals suited for in situ X-ray diffraction or Raman spectroscopic high-pressure studies in diamond anvil cells.

S2. Experimental

Starting materials for the flux growth experiments were dried reagent grade Tm₂O₃, SiO₂ and LiF. Due to the pronounced hygroscopicity of the alkali fluoride, sample preparation was performed in a glove bag under nitrogen atmosphere. 0.1 g of the nutrient consisting of a mixture of Tm₂O₃:SiO₂ in the molar ratio 1:4 was homogenized in an agate mortar with 0.1 g LiF. Subsequently, the educts were loaded into a platinum tube with an outer diameter of 3 mm and with 20 mm length. After sealing, the tube and its content were heated in a resistance furnace from 373 K to 1373 K with a rate of 50 K/h and isothermed for 2 h at the target temperature. The sample was cooled down to 1073 K with a rate of 5 K/h and, finally, the temperature was reduced to 373 K with a rate of 100 K/h. Removal of the flux with water left a residue of transparent, colorless, optically biaxial and highly birefringent crystals up to 500 µm in size. One of the optically biaxial crystals showing sharp extinction when observed between crossed polarizers was selected for further structural studies and mounted on the tip of a glass fiber using finger nail hardener as glue.

S3. Refinement

Similar sets of lattice parameters could be found in the recent WEB-based version of the Inorganic Crystal Structure Database (ICSD, 2014) for the chemically closely related thortveitite-type materials with composition (REE)₂Si₂O₇ pointing to an isostructural relationship, which was confirmed by the subsequent structure analysis by direct methods. For structure determination a data set corresponding to a hemisphere of reciprocal space was collected.

Figures

Fig. 1.

Representation of a single [Si2O7]-unit. Ellipsoids are drawn at the 60% probability level. [Symmetry codes: (i) x, y, -1 + z (ii) -x, y, -z (iii) -x, y, 1 - z (iv) x, -y, -1 + z].

Fig. 2.

Representation of the coordination around the trivalent Tm ion. Ellipsoids are drawn at the 60% probability level. [Symmetry codes: (i) 1 - x, y, 1 - z (ii) 1/2 + x, 1/2 - y, z (iii) 1/2 - x, 1/2 - y, 1 - z].

Fig. 3.

Single layer of edge-sharing octahedra and one of the two adjacent sheets containing [Si2O7]-units in a projection parallel to [001]. Red, grey and blue spheres represent oxygen, silicon and thulium ions.

Crystal data

Tm2Si2O7	F(000) = 444
Mr = 506.04	Dx = 6.021 Mg m−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 762 reflections
a = 6.8205 (14) Å	θ = 3.8–29.3°
b = 8.9062 (18) Å	µ = 31.99 mm−1
c = 4.6937 (11) Å	T = 293 K
β = 101.78 (2)°	Platy fragment, colourless
V = 279.11 (10) Å3	0.05 × 0.03 × 0.01 mm
Z = 2

Data collection

Agilent Xcalibur (Ruby, Gemini ultra) diffractometer	340 independent reflections
Radiation source: Enhance (Mo) X-ray Source	330 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
Detector resolution: 10.3575 pixels mm-1	θmax = 27.6°, θmin = 3.8°
ω scans	h = −8→8
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2014)	k = −11→11
Tmin = 0.231, Tmax = 1	l = −4→6
894 measured reflections

Refinement

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.018	w = 1/[σ2(Fo2) + (0.0265P)2] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.045	(Δ/σ)max < 0.001
S = 1.15	Δρmax = 1.62 e Å−3
340 reflections	Δρmin = −1.42 e Å−3
32 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.0072 (6)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in lengths, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tm	0.5	0.19345 (4)	0.5	0.0045 (2)
Si	0.2186 (3)	0	0.9130 (5)	0.0044 (5)
O1	0.3804 (9)	0	0.2130 (12)	0.0069 (12)
O2	0	0	0	0.0129 (19)
O3	0.2357 (6)	0.1505 (5)	0.7213 (9)	0.0073 (8)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Tm	0.0032 (3)	0.0043 (3)	0.0058 (3)	0	0.00056 (15)	0
Si	0.0049 (11)	0.0043 (11)	0.0042 (11)	0	0.0013 (9)	0
O1	0.006 (3)	0.007 (3)	0.005 (3)	0	−0.004 (2)	0
O2	0.008 (4)	0.019 (5)	0.011 (5)	0	0.001 (4)	0
O3	0.006 (2)	0.007 (2)	0.009 (2)	0.0042 (17)	0.0026 (17)	0.0045 (17)

Geometric parameters (Å, º)

Tm—O3i	2.217 (4)	Si—O3vi	1.632 (4)
Tm—O3ii	2.217 (4)	Si—O3	1.632 (4)
Tm—O1	2.236 (4)	O1—Sivii	1.602 (6)
Tm—O1iii	2.236 (4)	O1—Tmiii	2.236 (4)
Tm—O3	2.289 (4)	O2—Siviii	1.624 (2)
Tm—O3iv	2.289 (4)	O2—Sivii	1.624 (2)
Si—O1v	1.602 (6)	O3—Tmii	2.217 (4)
Si—O2v	1.624 (2)
O3i—Tm—O3ii	102.3 (2)	O3—Tm—O3iv	160.7 (2)
O3i—Tm—O1	154.9 (2)	O1v—Si—O2v	106.4 (2)
O3ii—Tm—O1	93.47 (17)	O1v—Si—O3vi	111.8 (2)
O3i—Tm—O1iii	93.47 (17)	O2v—Si—O3vi	108.14 (18)
O3ii—Tm—O1iii	154.9 (2)	O1v—Si—O3	111.8 (2)
O1—Tm—O1iii	79.2 (2)	O2v—Si—O3	108.14 (18)
O3i—Tm—O3	117.09 (17)	O3vi—Si—O3	110.4 (3)
O3ii—Tm—O3	75.78 (17)	Sivii—O1—Tm	129.10 (12)
O1—Tm—O3	85.4 (2)	Sivii—O1—Tmiii	129.10 (12)
O1iii—Tm—O3	79.74 (18)	Tm—O1—Tmiii	100.8 (2)
O3i—Tm—O3iv	75.78 (17)	Siviii—O2—Sivii	180.00 (14)
O3ii—Tm—O3iv	117.09 (17)	Si—O3—Tmii	130.3 (3)
O1—Tm—O3iv	79.74 (18)	Si—O3—Tm	122.6 (2)
O1iii—Tm—O3iv	85.43 (19)	Tmii—O3—Tm	104.22 (17)

Symmetry codes: (i) x+1/2, −y+1/2, z; (ii) −x+1/2, −y+1/2, −z+1; (iii) −x+1, −y, −z+1; (iv) −x+1, y, −z+1; (v) x, y, z+1; (vi) x, −y, z; (vii) x, y, z−1; (viii) −x, −y, −z+1.